Wind and seed: a conceptual model of shape-formation in the cushion plant Azorella Selago
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Wind and seed: a conceptual model of shape-formation in the cushion plant Azorella Selago Madeleine L. Combrinck & Thomas M. Harms & Melodie A. McGeoch & Janine Schoombie & Peter Christiaan le Roux
Received: 1 February 2020 / Accepted: 3 August 2020 # The Author(s) 2020
Abstract Aims The sub-Antarctic cushion plant, Azorella selago, is usually hemispherical when small but frequently crescent-shaped when larger. Spatial variation in wind speed and in air-borne seed and sediment deposition is examined to determine if wind scouring and deposition patterns could contribute to the development of nonhemispherical shapes in cushion plants. Methods Computational fluid dynamic analyses were conducted for hemispherical and crescent-shaped cushion plants parameterizing models with data from A. selago habitats on Marion Island. Numerical data
Responsible Editor: Jeffrey Walck M. L. Combrinck : T. M. Harms Department of Mechanical and Mechatronic Engineering, Stellenbosch University, Stellenbosch, South Africa M. L. Combrinck (*) Department of Mechanical and Construction Engineering, University of Northumbria, Newcastle upon Tyne, UK e-mail: [email protected] M. A. McGeoch School of Biological Sciences, Monash University, Clayton, Victoria 3800, Australia J. Schoombie : P. C. le Roux Department of Plant and Soil Sciences, University of Pretoria, Pretoria, South Africa P. C. le Roux Center for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa
were contextualized with field observations to arrive at a conceptual model for shape development. Results Airflow modelling showed that both wind scouring and seed deposition of the commonly cooccurring grass Agrostis magellanica are greater on the windward side of the plant. By contrast, heavier sediment particles are predominantly deposited on the leeward side of plants, leading to burial of lee-side A. selago stems. This sediment accumulation may initiate the development of the crescent-shape in hemispherical plants by increasing stem mortality on the plant’s leeward edge. Once developed, the crescent-shape is probably self-reinforcing because it generates greater air recirculation (and lower air velocities) which enhances further deposition and establishment of A. magellanica grasses in the lee of the crescent. The conceptual model consists therefore of three stages namely, (1) negligible air recirculation, (2) sediment deposition and grass establishment, and (3) differential cushion growth. Conclusion This conceptual model of plant shape development may explain the occurrence and orientation of crescent-shaped cushion plants and highlights how predicted changes in wind patterns may affect vegetation patterns.
Keywords Aeolian processes . Airflow modelling . Burial . Computational fluid dynamics . Positive feedback . Shading . Sub-Antarctic . Vegetation patterning
Plant Soil
Introduction Strong and prevailing winds can have considerable effects on the growth and survival patterns of indiv
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